The invention relates to a cell test for detecting and identifying a shorted thyristor, such as a silicon controlled rectifier, using current transformers.
One of the known cell test methods for detecting shorted thyristors, such as silicon controlled rectifiers (SCR), is by gating each cell individually and monitoring the resulting shunt current or voltage level across an external motor circuit. If the current or voltage level of the voltage is above a threshold value, then a failed cell is identified. A look-up table may be used to assist in the identification of the shorted cell based on which cell is gated.
However, there are various shortcomings to this technique. For example, when testing for a short using a shunt, variances in the motor circuit inductance and resistance will influence the shunt current level. As a result, too low a current signal level will result in a short not being detected, thus allowing the test to continue. Fuse failure can then result when paired cells are fired during the following open circuit test. Further, detection by voltage is sensitive to extraneous voltage spikes, i.e., ringing due to snubbers, and can result in a false detection occurring. The method of the invention addresses these and other problems with the known cell test techniques.
In accordance with one aspect of the invention, a method is provided for determining a shorted thyristor cell in a bridge that supplies a load from a source, the bridge including a plurality of the thyristor cells. The method includes the step of sequentially gating each of the cells to a conducting state, so that only one cell is gated at one time; providing at least one current transformer in the bridge; generating a current flow that passes through the bridge including the one cell that is gated; observing current in the at least one current transformer to determine a short in one of the cells the that is not gated; and determining a shorted cell based on the step of observing current in the at least one current transformer.
In accordance with a further aspect of the invention, a method is provided for determining a shorted thyristor cell in a bridge that supplies a load from a source, the bridge including a plurality of the thyristor cells, the method including the steps of sequentially gating each of the cells to a conducting state, so that only one cell is gated at one time; providing at least one current transformer in the bridge, the at least one current transformer being placed on incoming lines to the bridge, the bridge supplying the load to a motor; generating a current flow that passes through the bridge including the one cell that is gated, and wherein the bridge further includes a shunt, the shunt having a shunt current signal value, and wherein the current flow that is generated is below the shunt current signal value; observing current in the at least one current transformer to determine a short in one of the cells the that is not gated; and determining a shorted cell based on the step of observing current in the at least one current transformer.